Influenza viruses have two kinds of glycoproteins in their envelope membranes: hemagglutinin (HA) and sialidase (neuraminidase), each of which plays an important role in establishment of viral infection and viral budding from a host cell, respectively. Hemagglutinin recognizes a sialic acid-containing sugar chain generally present on the cell membrane of a host (a human or an animal other than a human, such as a mammal, a bird, a reptile, a fish, an amphibian, etc.) as a receptor to which it specifically binds, leading to intracellular endocytosis of the influenza virus. On the other hand, neuraminidase, a receptor-destroying enzyme, serves to cleave sialic acid residues on the host cell membrane or virus' own membrane when the viral particles bud or are released from the host cell.
Hemagglutinin, involved in the first step of influenza virus infection, has various subtypes based on the diversity of amino acid sequences of the antigen-determining regions (A-E) which are highly mutatable. Since amino acid sequence homologies among hemagglutinin subtypes are 25 to 75%, it is extremely difficult to develop an influenza vaccine based on antigenicity. On the other hand, the so-called receptor binding pocket region which binds to the receptor of a host cell is comparatively less mutatable and its three-dimensional structure is well conserved (Y. Suzuki, Prog. Lipid. Res., 33, 429 (1994). Therefore, to prevent influenza virus infection, if an agent inhibits the function of hemagglutinin, which is to contribute to establishment of infection, by specifically binding to it, broad effect will be expected in prevention of influenza virus infection, and thus development of such an agent has been demanded.
For example, based on the fact that hemagglutinin recognizes and binds to a sialic acid-containing sugar chain of a host receptor, a variety of hemagglutinin-binding sugar analogs have been obtained to date by using the technique of screening for sugar analogs specifically binding to the binding site of hemagglutinin (R. Roy, et al., J. Chem. Soc., Chem. Commun., 1869 (1993); M. Mammen, et al., J. Med. Chem., 38, 4179 (1995); T. Sato, et al., Chem. Lett., 145 (1999); M. Itzstein, et al., Nature, 363, 418 (1993)).
Another technique has been developed for preparing an antibody (anti-idiotype antibody) against the antigenic type (idiotype) of the antigen-binding site of a monoclonal antibody against the hemagglutinin receptor sugar chain. The principle applied here is that, in place of the three-dimensional structures of sialic acid and sialo-sugar chains acting as hemagglutinin receptors, the amino acid sequence of the supervariable region of an antiidiotype antibody which, in spatial configuration, resembles the three-dimensional structures, is constructed and caused to mimic the hemagglutinin receptors on host cells (Yasuo Suzuki, “Virus Kansen to Tosa” (Viral Infection and Sugar Chain), Nikkei Science supplemental issue “Tosa to Saibo” (Sugar Chain and Cell), pp. 89-101, October 1994].
However, although the receptor binding pocket region is comparatively less mutatable and its three-dimensional structure is well conserved, the region is specific to the subtype of hemagglutinin and its binding constant is not so high. Therefore, development of agents acting on influenza viruses in general, regardless of the subtype, has been awaited.
Then, 15-residue oligopeptides were screened by the phage display method for those which prevent influenza virus infection by binding to hemagglutinin, and 11 types (WO00/59932) and 3 types (Japanese Laid-Open Application No. 2002-284798; T. Sato, et al., Peptide Science 2001, 329 (2002)) of oligopeptides were identified (WO00/59932).